Method and System for Drilling with Reduced Surface Pressure

ABSTRACT

A drilling method includes arranging a dual drill string ( 20 ) having an inlet fluid conduit ( 9 ) and a return fluid conduit ( 10 ) in a well ( 14 ) formed in a subsurface formation such that a well annulus ( 22 ) is formed between the dual drill string ( 20 ) and a wall ( 21 ) of the well ( 14 ). A divider element ( 3 ) is disposed in the well annulus ( 22 ) to divide the well annulus ( 22 ) into an upper annular region ( 5 ) extending above the divider element ( 3 ) to a surface of the well annulus ( 22 ) and a lower annular region ( 12 ) extending below the divider element ( 3 ) towards a bottom of the well annulus ( 22 ). The lower annular region ( 12 ) contains a first fluid ( 25 ). A second fluid ( 27 ) is fed into the upper annular region ( 5 ). The method includes configuring the second fluid such that the density of the second fluid ( 27 ) is greater than the density of the first fluid ( 25 ).

FIELD

The present invention relates generally to a method and system fordrilling a well.

BACKGROUND

To extract hydrocarbon fluids from reservoirs in earth formations, wellsare drilled into the formations. The development of drilling techniqueshas evolved into the possibility of drilling wells in any direction suchas to extract as much as possible of the hydrocarbon fluids present inthe formations drilled. A well may for instance comprise a substantiallyvertical section and at least one section deviating from the verticalsection, possibly a substantially horizontal section. The sectionsdeviating from the substantially vertical section tend to be long, oftenextending for thousands of meters into a formation. To meet increasingdemand to add to energy reserves, hydrocarbon exploration is beingpushed into deeper waters, and the depths of wells are increasing.

Drilling is normally performed by inserting a drill bit on the end of adrill string into the well. The weight of the drill string isproportional to the length of the drill string. When drilling at largewater depths, the depth of the water also influences the pressureconditions in the well and adds to the weight of the drill string. Onedoes not want the formation fluids to penetrate the drilled well duringdrilling; therefore, the pressure exerted by the drilling system on theformation should be higher than the formation pore pressure. Drillingsystem should also be understood as including the fluid added betweenthe drill string and the unlined formation wall. With this one also hascontrol of the well during drilling and will therefore prevent blowoutsof the well. At the same time, there is also a need to limit the amountof drilling fluid that penetrate the unlined formation wall, and also aneed to prevent fracturing the side wall of the drilled bore beforeproduction starts. This gives that the pressure exerted by the drillingsystem must not exceed the fracturing pressure of the formation. Theformation pore pressure is also dependent on the hydrostatic column, andat larger water depths the formation pore pressure increases. When thepressure exerted by the drilling system moves towards the boundariesdefined by the formation pore pressure and formation fracturingpressure, the well has to be cased by casing or liner before drillingcan be continued. It is therefore a need to provide a method of drillingwhere the drilling can proceed for a longer period of time in theallowed pressure range between the formation pore pressure and theformation fracturing pressure, i.e., broader drilling window.

The term “drilling” should be understood to refer to establishing a holein the ground by the means of a drill string. It particularly appliesfor drilling in the crust of the earth for hydrocarbon recovery,tunnels, canals or for recovery of geothermal energy, both offshore andonshore.

WO 2010/039043 A1 describes a downhole well tool comprising a tool unit.The tool unit comprises at least one first fluid conduit and a returnfluid conduit, and the tool unit is arranged to be placed in well boredefining an annular space between the well unit and the well bore orcased well bore. The return fluid conduit may be arranged in the firstfluid conduit, leaving an annular space in between the first fluidconduit and the return fluid conduit for the flow of the first fluid,and wherein the return fluid conduit passes in the centrally arrangedspace of the return fluid conduit.

From document WO 94/13925 A1 it is known to drill with dual pipesarranged next to each other, where one pipe is used for the pumping offluids from the surface to the drill bit, and the other pipe serves as areturn line for the drilled cuttings and fluids from the bit to thesurface facility. At a lower part of the drill string, above the bit, isarranged a sealing piston. Above the piston is a seal closing the spacebetween the pipes and the borehole wall or casing, defining a volumebetween said piston and the seal. Pressurized fluid, such as hydraulicfluid, pumped into this volume, urges the piston, and thereby the bit,hydraulically against the bottom of the hole. When drilling in aformation, trying to reach the potential hydrocarbon reservoir, zoneshaving higher formation pore pressures than the surrounding formationmay be encountered. These zones might be pockets or reservoirs of highpressure gas or water. Drilling through such zones may be difficult, oreven impossible, by conventional drilling technologies as it is verydifficult to keep control on the pore pressure at the same time asdrilling underbalanced (UBD) or using managed pressure drilling (MPD)and still reaching further down the high pressure formation zone. UBD isreferred to as a drilling state where the hydrostatic pressure insidethe casing or liner is less than the reservoir pressure, while MPD is asuited method if the difference between the formation pore pressure andthe formation fracturing pressure is low. MPD is an adaptive drillingmethod used to more precisely control the annular pressure profilethroughout the wellbore.

To be able to drill, the weight of the drilling system has to be higherthan the formation pore pressure. It might be possible to pumppressurized fluid, such as hydraulic fluid, above the piston, but thenthe rotating control device (RCD), typically arranged at the top of thewell, and defining the upper boundary of an annular volume between thepiston and the top of the well, would have to withstand the pressuresfrom the pumped pressurized fluid. As the drill string rotates throughthe RCD with mud, there will always be a limited pressure/rotation rangefor these products.

SUMMARY

In one aspect of the present invention, a drilling method includesarranging a dual drill string having one inlet fluid conduit and onereturn fluid conduit in a well drilled in a formation such that a wellannulus is formed between the dual drill string and a wall of the well.The method includes arranging a divider element in the well annulus todivide the well annulus into an upper annular region, which extendsabove the divider element to a surface of the well annulus, and a lowerannular region, which extends below the divider element towards a bottomof the well annulus. The method includes feeding a second fluid having asecond density into the upper annular region. The feeding includesconfiguring the second fluid such that the second density is greaterthan the first density.

In one embodiment, at least a portion of the first fluid is fluid fromthe formation.

In one embodiment, feeding the second fluid includes providing thesecond fluid as an unpressurized fluid.

In one embodiment, the method further includes measuring pressure at ornear at least one of an upper surface of the divider element exposed tothe upper annular region and a lower surface of the divider elementexposed to the lower annular region.

In one embodiment, the method further includes using the measuredpressure to selectively adjust the second density such that the seconddensity remains greater than the first density during drilling of thewell.

In one embodiment, the method includes arranging a rotating control unitnear the surface of the well annulus. Pressure acting on the rotatingcontrol unit will be determined by pressure in the well and density ofthe fluid in the well annulus.

In another aspect of the present invention, a drilling system includes adual drill string having one inlet fluid conduit and one return fluidconduit. The dual drill string is arranged in a well formed in aformation such that a well annulus is formed between the drill stringand a wall of the well. A divider element arranged in the annulusdivides the well annulus into an upper annular region extending abovethe divider element to a surface of the well annulus and a lower annularregion extending below the divider element towards the bottom of thewell annulus. The lower annular region contains a first fluid having afirst density. A second fluid having a second density is disposed in theupper annular region, where the second density is greater than the firstdensity.

In one embodiment, the drilling system further includes a fluid inletthrough which the second fluid is fed into the upper annular region.

In one embodiment, the drilling system further includes a rotatingcontrol unit arranged near the surface of the well annulus, where therotating control unit is in communication with the fluid inlet.

In one embodiment, the drilling system further includes a blowoutpreventer arranged near the surface of the well annulus in connectionwith the rotating control unit, the blowout preventer forming an upperboundary of the upper annular region.

In one embodiment, the drilling system further includes means formeasuring pressure at or near a lower surface of the divider elementexposed to the lower annular region.

In one embodiment, the drilling system further includes means formeasuring pressure at or near an upper surface of the divider elementexposed to the upper annular region.

In one embodiment, the divider element is a piston.

In one embodiment, the divider element is fixed to the dual drillstring.

In another embodiment, the divider element is movable relative to thedual drill string.

In one embodiment, the two fluid conduits of the dual drill string areconcentric.

In one embodiment, at least a portion of the first fluid is fluid fromthe formation.

In one embodiment, the second fluid is unpressurized.

It is to be understood that both the foregoing summary and the followingdetailed description are exemplary of the invention and are intended toprovide an overview or framework for understanding the nature andcharacter of the invention as it is claimed. The accompanying drawingsare included to provide a further understanding of the invention and areincorporated in and constitute a part of this specification. Thedrawings illustrate various embodiments of the invention and togetherwith the description serve to explain the principles and operation ofthe invention.

BRIEF DESCRIPTION OF THE DRAWING

The following is a description of the figures in the accompanyingdrawings. The figures are not necessarily to scale, and certain featuresand certain views of the figures may be shown exaggerated in scale or inschematic in the interest of clarity and conciseness.

FIG. 1 shows a well with a drilling system according to one embodimentof the invention.

FIG. 2 is a graph showing pressure as a function of depth in a wellannulus for two different weight fluids.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details may beset forth in order to provide a thorough understanding of embodiments ofthe invention. However, it will be clear to one skilled in the art whenembodiments of the invention may be practiced without some or all ofthese specific details. In other instances, well-known features orprocesses may not be described in detail so as not to unnecessarilyobscure the invention. In addition, like or identical reference numeralsmay be used to identify common or similar elements.

The present invention relates to a method of controlling pressure in awell annulus such that pressure at a rotating control unit (RCD) nearthe well annulus is kept low. The method involves use of a dividerelement, typically a piston, arranged in the well annulus. Thearrangement is such that there is fluid below the divider element andfluid above the divider element, where the fluid above the dividerelement is a heavy fluid and the fluid below the divider element is alight fluid, i.e., the fluid above the divider element has a higherdensity than the fluid below the divider element. In FIG. 2, line Arepresents the pressure in the well annulus as a function of depth forthe system where light fluid is below the divider element and heavyfluid is above the divider element. For comparison, line B representsthe pressure in the well annulus as a function of depth if the fluidabove and below the divider element are both light fluids, e.g., havingsubstantially equal densities, or if there is no divider element in thewell annulus. As illustrated in FIG. 2, lines A and B have the samebottomhole pressure, as indicated at P1. Right below the dividerelement, at P2, the pressures for lines A and B are also the same. Abovethe divider element, line A diverges from line B. The surface pressuresof lines A and B are shown at P3 and P4, respectively. As shown, thesurface pressure for line A is lower than the surface pressure for lineP4. This is due to the heavy fluid above the divider element in thesystem represented by line A. This means that the RCD in the systemrepresented by line A will experience a lower surface pressure than theRCD in the system represented by line B. This reduction in the pressureacting on the RCD will enable a broader drilling window. If the heavyfluid in the system represented by line A is unpressurized, the pressureacting on the RCD can be as low as zero.

FIG. 1 shows a drilling system 1 according to one aspect of the presentinvention. The drilling system 1 would exhibit a pressure in the wellannulus similar to line A of FIG. 2, as described above. The drillingsystem 1 is shown in the context of offshore drilling, but it may alsobe applied to land drilling. A well 14 has been drilled throughformation 32 and is being drilled through a high pressure formation 34overlying a hydrocarbon formation 36. The upper part of the well 14 isprovided with casing 2. The lower part of the well 14 is not cased. Thedrilling system 1 consists of a drill string 20 having dual pipes. Thepipes can be concentric or positioned next to each other. In the shownembodiment the pipes are concentric. A first pipe 29 has an inlet fluidpath A connected to an inlet fluid conduit 10. A second pipe 30 has areturn fluid path B connected to a return fluid conduit 9. The returnfluid conduit 9 is on the inside of the inlet fluid conduit 10, but inan alternative embodiment the inlet fluid conduit 10 may be on theinside of the return fluid conduit 9. The lower part of the drill string20 has a bottom hole assembly (BHA) 15 and a drill bit 4 having adrilling fluid outlet 18 in its lower end. The BHA 15 may be providedwith a crossover valve 16. A cuttings inlet 17 is positioned on theupper part of the BHA 15.

The drill string 20 is arranged in the well 14 such that a well annulus22 is formed between the drill string 20 and a wall 21 of the well 14. Adivider element 3, such as a piston, plunger or ram 3, is arranged inthe well annulus 22 on the outside of the drill string 20. The dividerelement 3 divides the well annulus 22 into an upper annular region 5above the divider element 3 and a lower annular region 12 below thedivider element 3. The upper annular region 5 extends from an uppersurface 3 b of the divider element 3 to the surface of the well annulus22, while the lower annular region 12 extends from a lower surface 3 aof the divider element 3 towards the bottom of the well annulus 22. Inone embodiment, the lower annular region 12 extends all the way to thebottom of the well 14. The divider element 3 can be set in an area ofthe well 14 with casing 2 or in open hole. In one embodiment, thedivider element 3 is fixed to the drill string 20. In this case, if theforce acting on the upper surface 3 b of the divider element 3 isgreater than the force acting on the lower surface 3 a of the dividerelement 3, the divider element 3 will tend to move downwards. If the netforce on the divider element 3 overcomes the weight of the drill string20, the drill string 20 will be urged downwards, i.e., towards thebottom of the well 14, by motion of the divider element 3. In anotherembodiment, the divider element 3 is not fixed to the drill string 20and is free to move relative to the drill string 20.

Near the surface of the well 14 or well annulus 22, in the area close tothe sea floor 13, is arranged a blowout preventer (BOP) 8 and a rotatingcontrol device (RCD) 7. The RCD 7 is in communication with a tank (notshown) or similar storing facility for storage of fluid, such asdrilling fluid, through a fluid inlet 6. The fluid inlet 6 also leads tothe upper annular region 5 defined above the divider element 3. A topdrive adapter 11, for rotating or driving the drill string, is arrangedat a surface vessel or platform (not shown).

When performing drilling operations in the well 14, the drilling system1 also includes a lower annular region fluid 25 contained in the lowerannular region 12 and an upper annular region fluid 27 disposed in theupper annular region 5. In one embodiment, at least a portion of thelower annular region fluid 25 is from the formation(s) in which the well14 is drilled. The remainder of the lower annular region fluid 25 may befrom fluid discharged from the drill string 20 into the bottom of thewell 14. The lower annular region fluid 25 will apply a first pressureon the lower surface 3 a of the divider element 3.

The upper annular region fluid 27 may be fed into the upper annularregion 5 through the fluid inlet 6. The upper annular region fluid 27will apply a second pressure on the upper surface 3 b of the dividerelement 3. The upper annular region 5 may be filled partially orentirely with the upper annular region fluid 27. The pressure at thesurface of the column of fluid in the upper annular region 5 willdetermine the pressure at the surface of the well annulus 22. Thepressure acting on the RCD 7 will be determined by the pressure in thewell 14 and the density of fluid in the well annulus 22, which isrelated to the pressure in the well annulus 22.

In one embodiment, the density of the upper annular region fluid 27 isgreater than the density of the lower annular region fluid 25. In thiscase, the hydrostatic pressure at the upper surface 3 b of the dividerelement 3 will be greater than the hydrostatic pressure at the lowersurface 3 a of the divider element 3. In one embodiment, the upperannular region fluid 27 is “unpressurized,” i.e., does not have raisedpressure that is produced or maintained artificially. If the upperannular region fluid 27 is unpressurized, the pressure at the surface ofthe well annulus 22 and acting on the RCD 7 will be essentially zero.The upper annular region fluid 27 may be a drilling fluid, for example.In general, the upper annular region fluid 27 can be a liquid, a mixtureof one or more liquids, or a mixture of one or more liquids and one ormore types of solid particulates. The composition of the upper annularregion fluid 27 will be selected to achieve a desired density, whichwould preferably be greater than that of the lower annular region fluid25. Typically, the density of the upper annular region fluid 27 will begreater than 1.0 kg/litre.

In one embodiment, a pressure sensor 24 is arranged at or near the uppersurface 3 b of the divider element 3 to measure pressure at or near theupper surface 3 b. Alternately, or additionally, a pressure sensor 26may be arranged at or near the lower surface 3 a of the divider element3 to measure pressure at or near the lower surface 3 a. As mentionedearlier, the density of the upper annular region fluid 27 needs to begreater than the density of the lower annular region fluid 25. If thedensity of the upper annular region fluid 27 is greater than the densityof the lower annular region fluid 25, the pressure measured at or nearthe upper surface 3 b of the divider element 3 will be greater than thepressure measured at or near the lower surface 3 a of the dividerelement 3. If the outputs of the sensors 24, 26 indicate that thedensity of the upper annular region fluid 27 is not greater than thedensity of the lower annular region 25, the density of the upper annularregion fluid 27 can be increased. Monitoring of pressure at or near thesurfaces 3 a, 3 b may be carried out at various times during thedrilling process. This is because the conditions in the lower annularregion 12 can change at any time, e.g., due to formation fluid influx orchange in the composition of the fluid pumped down the drill string 20.Adjustment of the density of the upper annular region fluid 27 may bemanual or automated.

The method of reducing the pressure acting on the RCD 7 through use of aheavy fluid above the divider element 3, as described above, can be usedwith any drilling mode, such as underbalanced, managed pressure, andoverbalanced drilling modes. This means that selection of the density ofthe upper annular region fluid 27 may be influenced by the formationpore pressure.

Although the present invention is described in terms of some preferredembodiments, alterations can be made. For example, the fluid flowdirections, and the inlets and outlets of the cuttings and drillingfluid, can be swapped. A person skilled in the art will understand thatthere are other alterations and modifications that could be made thatare within the scope of the invention as defined in the attached claims.

1. A drilling method, comprising: arranging a dual drill string havingone inlet fluid conduit and one return fluid conduit in a well drilledin a formation such that a well annulus is formed between the dual drillstring and a wall of the well; disposing a divider element in the wellannulus to divide the well annulus into an upper annular region ,extending above the divider element, to a surface of the well annulusand a lower annular region extending below the divider element towardsthe bottom of the well annulus, the lower annular region containing afirst fluid having a first density; and feeding a second fluid having asecond density into the upper annular region, the feeding comprisingconfiguring the second fluid such that the second density is greaterthan the first density.
 2. A method according to claim 1, wherein atleast a portion of the first fluid is fluid from the formation.
 3. Amethod according to claim 1, further comprising providing the secondfluid as an unpressurized fluid.
 4. A method according to claim 1,further comprising measuring pressure at or near at least one of anupper surface of the divider element exposed to the upper annular regionand a lower surface of the divider element exposed to the lower annularregion.
 5. A method according to claim 1, further comprising using themeasured pressure to selectively adjust the second density such that thesecond density remains greater than the first density during drilling ofthe well.
 6. A method according to claim 1, further comprising arranginga rotating control unit near the surface of the well annulus, whereinpressure acting on the rotating control unit is determined by pressurein the well and density of fluid in the well annulus.
 7. A drillingsystem comprising: a dual drill string arranged in a well formed in aformation such that a well annulus, is formed between the dual drillstring and a wall of the well, the dual drill string having one inletfluid conduit and one return fluid conduit; a divider element arrangedin the well annulus, to divide the well annulus into an upper annularregion extending above the divider element to a surface of the wellannulus and a lower annular region below the divider element towards abottom of the well annulus, the lower annular region containing a firstfluid having a first density; and a second fluid having a second densitydisposed in the upper annular region, the second fluid being configuredsuch that the second density is greater than the first density.
 8. Adrilling system according to claim 7, further comprising a fluid inletthrough which the second fluid is fed into the upper annular region. 9.A drilling system according to claim 8, further comprising a rotatingcontrol unit arranged near the surface of the well annulus the rotatingcontrol unit being in communication with the fluid inlet.
 10. A drillingsystem according to claim 9, further comprising a blowout preventerarranged near the surface of the well annulus in connection with therotating control unit, the blowout preventer forming an upper boundaryof the upper annular region.
 11. A drilling system according to claim 7,further comprising means for measuring pressure at or near a lowersurface of the divider element exposed to the lower annular region. 12.A drilling system according to claim 7, further comprising means formeasuring pressure at or near an upper surface of the divider elementexposed to the upper annular region.
 13. A drilling system according toclaim 7, wherein the divider element is a piston.
 14. A drilling systemaccording to claim 7, wherein the divider element is fixed to the dualdrill string.
 15. A drilling system according to claim 7, wherein thedivider element is movable relative to the dual drill string
 16. Adrilling system according to claim 7, wherein the two fluid conduits ofthe dual drill string are concentric.
 17. A drilling system according toclaim 7, wherein at least a portion of the first fluid is fluid from theformation.
 18. A drilling system according to claim 7, wherein thesecond fluid is unpressurized.